OHAI-529: ip_scopes prefers ethernet to ppp

On hosts with virtual interfaces on RFC1918 networks, the ip_scopes
plugin may set `node.privateaddress` to a virtual address. This can
cause problems with automation that relies on the `privateaddress`
attribute.

This patch prefers non-ppp interfaces when setting `privateaddress`,
though it will use ppp interfaces if they are the only ones.